Personal computers and work stations in recent years are provided with a magnetic recording device having a large capacity and yet a small size. Higher recording density of magnetic disks is needed to produce such devices. Present magnetic recording systems employ a longitudinal magnetic recording system, where the axis of easy magnetization is parallel to the plane of the magnetic recording medium. Enhancing recording density in this longitudinal magnetic recording system needs reduction of a product of remanent magnetization Br and thickness t of the magnetic film of the recording medium, as well as increase in coercive force Hc. Accordingly, efforts are being made to reduce the film thickness and control the grain size of the magnetic film.
The longitudinal magnetic recording system, however, has a problem of decrease in readback signal output due to decrease of residual magnetic flux density caused by enhancement of demagnetizing field, which results from shortened length of a bit. In addition, a problem of thermal fluctuation becomes significant with minimization of grain size and reduction of thickness of the magnetic film. In these circumstances, there is a perception that achieving higher density is technically difficult in the magnetic disks based on the longitudinal magnetic recording system.
On the other hand, to solve these problems and to improve areal recording density, a perpendicular magnetic recording system is being contemplated. A magnetic recording medium based on the perpendicular magnetic recording system is designed so that the axis of easy magnetization of the magnetic film orients perpendicularly to the substrate surface. Accordingly, adjacent magnetization in a transition region of magnetization does not oppose each other, and even short bit length preserves stable magnetization and magnetic flux does not decrease. Thus, the perpendicular magnetic recording system is appropriate to a magnetic recording method for a high density magnetic recording medium.
The perpendicular magnetic recording medium, despite these advantages, involves a disadvantage of high media noise due to relatively large magnetic interaction between magnetic particles in the magnetic layer, which is caused by insufficient segregation of nonmagnetic substance to the grain boundary region. Therefore, it is required to develop a material control technique for promoting the grain boundary segregation of nonmagnetic substance, to reduce media noise and enhance SN ratio. High recording density has to be achieved accompanying the reduction of noises and improvement of the SN ratio.
A structure of a known perpendicular magnetic recording medium comprises a soft magnetic backing layer formed on a nonmagnetic substrate of aluminum or glass, an underlayer for perpendicular orientation of a magnetic layer formed on the soft magnetic backing layer, a perpendicular magnetic recording layer on the underlayer, and a protective layer formed thereon. Such a medium is called “a double layer perpendicular magnetic recording medium.” See, for example, Japanese Unexamined Patent Application Publication No. 2002-203306. Studies have been made on a perpendicular magnetic recording layer, including perpendicular magnetization films of cobalt based alloys such as Co—Cr, Co—Cr—Ta, and Co—Cr—Pt, multilayered lamination perpendicular magnetization films such as Pt/Co and Pd/Co, and amorphous perpendicular magnetization films such as Tb—Co and Tb—Fe—Co. Among them, the multilayered lamination perpendicular magnetization films including Pt/Co and Pd/Co exhibit large perpendicular magnetic anisotropy, high thermal stability, large coercive force, and is easier to achieve a squareness ratio approximately 1.0. Therefore, this type of perpendicular magnetization film is being studied extensively in an attempt to create a next generation recording medium having a high recording density. See, for example, Japanese Unexamined Patent Application Publication Nos. 2002-025032 and 2001-155329.
The reduction of media noise in a perpendicular magnetic recording medium having known multilayer structure is, however, not satisfactory yet, and further reduction of media noise and further improvement in read-write characteristics are required. Accordingly, there is a need for a perpendicular magnetic recording medium having high recording density and excellent read-write characteristics, with lower media noise.